Energy now lost as heat during the production of electricity could be harnessed through the use of silicon nanowires synthesized via a technique developed by researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley. The far-ranging potential applications of this technology include the DOE’s hydrogen fuel cell–powered “Freedom CAR” and personal power-jackets that could use heat from the human body to recharge cell phones and other electronic devices.
“For example, if it is cold outside and you are wearing a jacket made of material embedded with thermoelectric modules, you could recharge mobile electronic devices off the heat of your body [Figure 1],” explained Arun Majumdar, a mechanical engineer and materials scientist with joint appointments at Berkeley Lab and UC Berkeley. “In fact, thermoelectric generators have already been used to convert body heat to power wrist watches.”
Nano generators. Rough silicon nanowires demonstrated high-performance thermoelectric properties even at room temperature when connected between two suspended heating pads. Here, one pad serves as the heat source (pink) and the other as the sensor. Source: Lawrence Berkeley National Laboratory
“We’ve shown that it’s possible to achieve a large enhancement of thermoelectric energy efficiency at room temperature in rough silicon nanowires that have been processed by wafer-scale electrochemical synthesis,” said chemist Peidong Yang, the other principal investigator behind this research, who also holds joint Berkeley Lab and UC Berkeley appointments.
The researchers describe a unique “electroless etching” method by which arrays of silicon nanowires are synthesized in an aqueous solution on the surfaces of wafers that can measure dozens of square inches in area. The technique involves the galvanic displacement of silicon through the reduction of silver ions on a wafer’s surface. Unlike other synthesis techniques, which yield smooth-surfaced nanowires, this electroless etching method produces arrays of vertically aligned silicon nanowires that feature exceptionally rough surfaces. The roughness is believed to be critical to the surprisingly high thermoelectric efficiency of the silicon nanowires (Figure 2).
Miracle fibers. A cross-sectional scanning electron microscope image of an array of rough silicon nanowires includes an inset showing a typical wafer chip of these wires (a). The second photograph is a transmission electron microscope image of a segment of one of these wires in which the surface roughness can be clearly seen. The inset shows that the wire is single-crystalline all along its length. Source: Lawrence Berkeley National Laboratory
“Thermoelectric materials, which have the ability to convert heat into electricity, potentially could be used to capture much of the low-grade waste heat now being lost and convert it into electricity,” said Majumdar. “The same devices can also be used as refrigerators and air conditioners, and because these devices can be miniaturized, it could make heating and cooling much more localized and efficient.”
The ability to dip a wafer into solution and grow on its surface a forest of vertically aligned nanowires that are consistent in size opens the door to the creation of thermoelectric modules that could be used in a wide variety of situations. For example, such modules could convert the heat from automotive exhaust into supplemental power for a vehicle, or provide the electricity a conventional vehicle needs to run its radio, air conditioner, and power windows.
When scaled up, thermoelectric modules could eventually be used for cogenerating power with gas or steam turbines. “You can siphon electrical power from just about any situation in which heat is being given off, heat that is currently being wasted,” said Majumdar.
The Berkeley Lab researchers will be studying the physics behind this phenomenon to better understand and possibly manipulate it for additional improvements. Berkeley Lab’s Technology Transfer Department is now seeking industrial partners to further develop and commercialize this technology.